An IBlockPattern is an interface that allows for combining several blocks into one object.
It is comparable to what the IIngredient Interface is to IItemStacks.

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import.
import crafttweaker.block.IBlockPattern;

Technically, each time you call an IBlock object, you call an IBlockPattern object.
But there are cases when you explicitly get an IBlockPattern Object as return.

• OR two IBlocks

ZenGetter	What does it do	Return Type
ZenGetter blocks	What does it do Lists all possible blocks for this object	Return Type List<IBlock>
ZenGetter displayName	What does it do Returns the displayNames of the fitting blocks	Return Type string

You can OR two IBlockPattern Objects using the OR | Operator

You can check if an IBlockPatternObject contains another using the in keyword.
For example, you could check if a Block is in an IBlockPattern.